1. Field of the Invention
This invention relates to directional couplers in general and more particularly to a directional coupler having a small package size.
2. Description of the Prior Art
A directional coupler is a four port device. A directional coupler separates signals based on the direction of signal propagation. These devices are used to unequally split the signal flowing in the mainline. Referring to FIG. 1, a schematic diagram of a directional coupler is shown. Directional coupler 20 has an input port 22, an output port 24 and a coupled port 26. A termination port 28 is connected to a capacitor C and resistor R that are connected to ground. In an ideal situation some portion of the signal flowing into the input port 22 will appear at coupled port 26. Likewise any signal flowing into coupled port 26 will be coupled to input port 22. Output port 24 and coupled port 26 are isolated in that any signal flowing into port 24 will not appear at port 26 but will feed through to port 22. The RF directional coupler 20 is usually realized by two transformers 30 with a ferrite core that are connected.
Directional couplers have been made with ferrite transformers along with appropriate resistors and capacitors arranged around the ferrite transformer. These couplers provide multi-decade bandwidth. The directional coupler components are packaged on a printed circuit board. The transformer, resistors and capacitors are mounted to the printed circuit board. In some applications printed circuit board space can be extremely limited with additional space just not available. Unfortunately placing resistors and capacitors beside each transformer complicates the assembly program followed by the automated pick and place surface mount assembly equipment. This leads to lower production by the assembly machinery.
While directional couplers have been used, they have suffered from taking up excessive printed circuit board space and in being difficult to assemble. A current unmet need exists for a directional coupler that takes up less printed circuit board space and that can be easily assembled.
It is a feature of the invention to provide a directional coupler having a small package size that has repeatable electrical characteristics.
Another feature of the invention is to provide a directional coupler that includes a low temperature co-fired ceramic (LTCC) substrate. The LTCC substrate has several layers. Electrical components such as resistors and capacitors are integrated internal within the LTCC substrate. A transformer is attached to the upper layer of the LTCC substrate and is electrically connected to the resistors and capacitors. The transformer provides directional coupling. The LTCC substrate has electrically conductive vias extending therethrough. The vias are used to make electrical connections between layers of the LTCC substrate.
Another feature of the invention is to provide a directional coupler that takes up less printed circuit board space and has improved electrical repeatability.
A further object of the invention is to provide a method of manufacturing a miniature directional coupler.
The invention resides not in any one of these features per se, but rather in the particular combination of all of them herein disclosed and claimed. Those skilled in the art will appreciate that the conception, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention.